This invention relates to a photomask blank and a photomask prepared therefrom which are used in the manufacture of such semiconductor devices as integrated circuits (IC) and large scale integrated circuits (LSI), and more particularly to a chromium photo mask having a low reflection of the rear surface.
Among prior art photomasks there may be mentioned an emulsion mask wherein an emulsion capable of manifesting a high degree of resolution is coated on a transparent substrate made of glass or the like, and a hard mask wherein a film of chromium, chromium oxide, iron oxide etc. are deposited on the transparent substrate by evaporation or sputtering. Due to a recent tendency for increasing circuit elements to be included in the integrated circuit, hard masks are used in most cases.
A chromium mask is a typical use of the hard masks and comprises a transparent substrate and chromium film having a thickness of from 50 to 100 m.mu. which is deposited on the substrate by evaporation or sputtering.
The chromium mask has an extremely high reflection (50-60%) so that when printing a mask pattern on a wafer with the use thereof, there is a defect that the resolution of the pattern is decreased due to multiple reflections between the mask and the wafer. For this reason, surface reflection free chromium masks have been developed and used extensively, in which a chromium oxide film having a thickness of 25-35 m.mu. is formed on the chromium film.
In the surface reflection free chromium mask, the multiple reflections described above are alleviated or eliminated by the presence of the chromium oxide film so that resolution of the circuit pattern formed can be improved, but the alignment accuracy of the mask is not improved. More particularly, even with such a chromium mask, the reflection of the rear surface (the substrate side surface) of the target pattern that is formed is comparable with the percentage of surface reflection of a conventional wafer with the result that where a certain type of the auto-aligner which utilizes light which is transmitted through the substrate, or a non-contact printer, is used as an aligning device, it is impossible to obtain a sufficient contrast between the target pattern and the wafer.
To solve this problem, that is to decrease the rear surface reflection of the chromium mask, an improved hard mask has been proposed in which a chromium oxide film similar to the surface reflection prevention film and having a thickness of 30 to 40 m.mu. is interposed between the substrate and the chromium film. Such hard mask has a construction of a substrate - chromium oxide film - chromium film - chromium oxide film.
We have found that this hard mask incorporated with a rear surface reflection preventing film made of chromium oxide has the following disadvantages.
1. As the amount of side etching (corrosion of the film progressing parallel to the surface of the substrate) at the time of wet etching is large so that it is difficult to form clearly defined sharp edges for the pattern. This is caused by the difference in the solubilities of the chromium film and chromium oxide film in the etching solution. Although it is possible to considerably vary the solubility of the chromium oxide film by controlling the method of preparing the same, the condition of vapor deposition, etc., it is difficult to make the solubility of the chromium oxide film equal to that of the chromium film.
The chromium oxide film, which chromium oxide film is for achieving anti-reflection at the surface which is remote from the substrate, is formed on the chromium mask film, so that it is first dissolved. Accordingly, this chromium oxide film has little effect upon the side etching and the sharpness of the pattern. Where the solubility of the chromium oxide film which is utilized to prevent the rear surface reflection is lower than that of the chromium film, the side etching of the chromium mask film proceeds while the chromium oxide film is being dissolved. On the other hand where the solubility of the chromium oxide film is higher than that of the mask chromium film, the side etching of the chromium oxide film proceeds with the result that the chromium film of the mask becomes irregular, thus degrading the sharpness of the side etching and hence the quality of the resulting photomask.
Considering the problem from the standpoint of the theory of growth of the film by evaporation or sputtering at the initial stage of film forming, since the nuclear density of the chromium oxide film is smaller than that of the chromium film and at the film thickness of about 30 microns, the film structure of the former becomes coarser than that of the latter. These facts also cause blur or an irregular pattern edge and large quantity of side etching.
2. The effect of preventing reflection is not sufficiently large.
Although it is possible to make the reflection sufficiently low near a specific wavelength which is determined by the thickness of the chromium oxide film, it is impossible to make the reflection sufficiently low over the entire range of the visible wavelength. In other words, the light source utilized for auto-alignment is not limited to any type, so that it is desirable that the reflection be sufficiently low for light of any wavelength. However, it is impossible to fulfil such a requirement.
As can be clearly understood from the theory of interference, in order to decrease the reflection on the long wavelength side, it is essential to make the thickness of the chromium oxide film to be approximately 40 to 45 m.mu., but with such large film thickness the quantity of the side etching increases.
It is the principal object of this invention to provide an improved photomask blank and a photomask prepared therefrom having a low reflection at the rear surface (the substrate side surface) over substantially the entire range of the visible light, and a small quantity of side etching at the time of photoetching, which can prevent rear surface reflection.
According to this invention there is provided a photomask blank of the type comprising an optically transparent substrate, and a mask layer composed of chromium, characterized in that a film made of chromium, the chromium film having a thickness of less than 15 m.mu., and an intermediate layer containing chromium oxide are interposed between the substrate and the mask layer, wherein the chromium film and the intermediate layer are formed in this order from the substrate. The intermediate layer may comprise a single chromium oxide film, or a composite layer including a chromium oxide film and a chromium film or a layer comprising a mixture of chromium oxide and chromium.
The photomask of this invention is prepared by subjecting the mask layer, the chromium film and the intermediate layer of the photomask described above to an etching process.